1. Field of the Invention
This invention relates to a method of synthesizing energetic thermoplastic elastomers which are useful as binders of high energy compositions, such as rocket motor propellants, gun propellants, explosive munitions, gas generants of vehicle supplemental restraint systems, or the like.
2. Description of the Related Art
Solid high energy compositions, such as propellants, explosives, gas generants, and the like comprise solid particulates, such as fuel particulates and/or oxidizer particulates, dispersed and immobilized throughout a polymeric binder matrix.
Conventional solid composite propellant binders utilize cross-linked elastomers in which prepolymers are cross-linked by chemical curing agents. As outlined in detail in U.S. Pat. No. 4,361,526 to Allen, there are important disadvantages to using cross-linked elastomers as binders. Cross-linked elastomers must be cast within a short period of time after addition of the curative, which time period is known as the “pot life.” Disposal of a cast, cross-linked propellant composition is difficult, and usually is accomplished by burning, which poses environmental problems. Furthermore, current state-of-the-art propellant compositions have serious problems that include their use of nonenergetic binders which have lower performance and high end-of-mix viscosities.
In view of the inherent disadvantages associated with the use of cross-linked elastomeric polymers as binder materials, there has been considerable interest in developing thermoplastic elastomers suitable as binders for solid, high energy compositions. However, many thermoplastic elastomers fail to meet important requirements expected of propellant formulations, particularly the requirement of being processible below about 120° C., it being desirable that a thermoplastic elastomeric polymer for use as a binder in a high energy system have a melting temperature of between about 60° C. and about 120° C. The melting temperature is desirably at least about 60° C. because the propellant composition may be subject to somewhat elevated temperatures during storage and transport, and significant softening of the propellant composition at such elevated temperatures is unwanted. The setting of the melting temperature at not more than about 120° C. is determined by the instability, at elevated temperatures, of many components which ordinarily go into high energy compositions, particularly oxidizer particulates and energetic plasticizers. Many thermoplastic elastomers exhibit high melt viscosities which preclude high solids loading and many show considerable creep and/or shrinkage after processing. Thermoplastic elastomers typically obtain their thermoplastic properties from segments that form glassy domains which may contribute to physical properties adverse to their use as binders. Cross-linkable thermoplastic elastomers are block copolymers with the property of forming physical cross-links at predetermined temperatures. One thermoplastic elastomer, e.g., KratonTM, brand TPE, obtains this property by having the glass transition point of one component block above room temperature. At temperatures below 109° C., the glassy blocks of KratonTM form glassy domains and thus physically cross-link the amorphous segments. The strength of these elastomers depends upon the degree of phase separation. Thus, it remains desirable to have controlled, but significant, immiscibility between the two types of blocks, which is a function of their chemical structure and molecular weight. On the other hand, as the blocks become more immiscible, the melt viscosity increases, thus having a deleterious effect on the processibility of the material.
The above-mentioned U.S. Pat. No. 4,361,526 proposes a thermoplastic elastomeric binder which is a block copolymer of a diene and styrene, the styrene blocks providing a meltable crystal structure and the diene blocks imparting rubbery or elastomeric properties to the copolymer. The ‘526 patent states that this polymer is processed with a volatile organic solvent. Solvent processing is undesirable inasmuch as the dissolved composition cannot be cast in a conventional manner, e.g., into a rocket motor casing. Furthermore, solvent-based processing presents problems with respect to removal and recovery of solvent.
The preparation of energetic thermoplastic elastomers prepared from polyoxetane block copolymers has been proposed in U.S. Pat. No. 4,483,978 to Manser (“the ‘978 Patent”), and U.S. Pat. No. 4,806,613 to Wardle (“the ‘613 patent”), the complete disclosures of which are incorporated herein by reference to the extent that these disclosures are compatible with this invention. According to the latter, these materials overcome the disadvantages associated with conventional cross-linked elastomers such as limited pot-life, high end-of-mix viscosity, and scrap disposal problems.
The thermoplastic materials proposed by the ‘613 patent involve elastomers having both A and B blocks, each derived from cyclic ethers, such as oxetane and oxetane derivatives and tetrahydrofuran (“THP”) and THF derivatives. The monomer or combination of monomers of the A blocks are selected for providing a crystalline structure at usual ambient temperatures, such as below about 60° C., whereas the monomer or combination of monomers of the B blocks are selected to ensure an amorphous structure at usual ambient temperatures, such as above about −20° C. Typical of these materials is the random block copolymer (poly(3-azidomethyl-3-methyloxetane)-poly(3,3-bis(azidomethyl)oxetane), also known as poly(AMMO/BAMO). These block copolymers have good energetic and mechanical properties. Additionally, the block copolymers can be processed without solvents to serve as binders in high performance, reduced vulnerability explosive, propellant, and gas generant formulations. Advantageously, the block copolymers exhibit good compatibility with most materials used in such energetic formulations.
However, the processing techniques disclosed in the ‘978 and ‘613 patents involve the use of halogenated solvents, such as methylene chloride. Several drawbacks have been associated with the use of the halogenated solvents disclosed in the ‘978 and ‘613 patents. One drawback is the detrimental impact that halogenated solvents have on the environment. Another drawback of halogenated solvents is attributable to the additional drying steps which the pre-polymer blocks are subject to after their formation. The pre-polymer blocks are typically dried either with chemical drying agents, e.g., desiccants followed by filtration or by the azeotropic removal of water. The azeotropic removal of water is performed with toluene, which is different from the solvent selected for linking the pre-polymer blocks. The performance of an additional drying step and the use of different solvents in the azeotropic drying step and the linking step complicate processing and increase overall processing time. Moreover, toluene does not completely dissolve the end-capped blocks prior to the linking reaction and can interfere with the end-capping and linking catalysts. Yet another drawback associated with halogenated solvents is the relatively low concentrations of pre-polymer blocks and linking compounds that may be loaded in halogenated solvents for processing. The loading of the thermoplastic elastomer ingredients is limited by the solubility of the ingredients in the solvent. For example, the currently used process for forming thermoplastic elastomers by linking energetic polyether diols and diisocyanates typically use approximately 30–40% by weight solutions of the reactants in dichloromethane and 0.1% by weight tin catalyst. Additionally, completion of the reaction in halogenated solvents typically takes several days to a week.
U.S. Pat. No. 4,393,199 to Manser (“the ‘199 Patent”) describes the use of a non-halogenated solvent, nitromethane, during cationic polymerization of cyclic ethers. However, it has been found that cyclic ether pre-polymer blocks are not sufficiently soluble in nitromethane to adequately link the pre-polymer blocks once they are formed.
It would therefore be a significant advancement in the art to provide a synthesis route to making energetic thermoplastic elastomer binders which avoids the drawbacks associated with halogenated solvents while reducing processing time and increasing productivity.